Field of the Disclosure
The present disclosure relates generally to a dynamically biased baseband current amplifier, and more particularly, to a dynamically biased baseband current amplifier where a bias current is automatically adjusted based on a signal swing to improve power-efficiency.
Description of the Related Art
Electronic systems used in the fields of wireless communication include 2nd Generation (2G), 3rd Generation (3G), 4th Generation (4G) cellular radio integrated circuits, Wireless Fidelity (WiFi), Bluetooth, Zigbee radio integrated circuits, etc. Such systems include a baseband section that follows a Radio Frequency (RF) down-converter in a receiver or precedes an RF up-converter in a transmitter.
A baseband section consists of two identical paths, one for an in-phase stream (I-stream) and one for a quadrature-phase stream (Q-stream).
Integrated multi-mode multi-band transmitters (TXs) must meet diverse specifications related to output power, spectral regrowth, spurious emissions, out-of-band noise, and gain range, while occupying a small integrated circuit area and maintaining a high power-efficiency. Such transmitters are required to process constant envelope signals in the case of 2G Gaussian Minimum Shift Keying (GMSK), as well as high Peak-to-Average-Power-Ratio (PAPR) signals in the case of 4G Long Term Evolution (LTE), where the use of Orthogonal Frequency Division Multiplexing (OFDM) and complex modulation schemes cause the PAPR to exceed 6 dB.
Within 4G LTE20, the baseband signal can be either wideband, where an 18 MHz channel is fully occupied by 100 Resource Blocks (RBs) of 180 kHz bandwidth (100 RB/Full RB), or narrowband, where all of the signal power is concentrated in a single RB (1RB). When a single or few RBs are transmitted close to a channel edge, the TX nonlinearity leads to the generation of third-order and higher-order counter-intermodulation products (e.g., Third-Order Counter-Intermodulation (CIM3), Fifth-Order Counter-Intermodulation (CIM5), etc.) in the adjacent bands, causing the TX to fail spurious emissions specifications.
Different architectures have been proposed to meet these specifications. In a passive-mixer plus Driver Amplifier (DA) based architecture, the DA worsens the CIM terms generated by the passive mixer. CIM3 can be improved by removing the DA by utilizing a current-mode power-mixer. However, such a traditional, power-mixer based TX is biased in Class-A mode (i.e., fixed bias current).
In a fixed-bias system, signal-path circuitry is biased with a fixed current that is sufficiently high to pass a peak signal swing with good linearity. As the PAPR rises, peak signal swings occur less frequently, and a fixed bias current system unnecessarily wastes power by always being ready to process a peak signal swing that does not always occur.
In addition, Envelope-Tracking (ET) is a technique that is used to adaptively bias RF Power Amplifiers (PA)/Drive Amplifiers (DA). ET PAs require a fast and highly-linear supply modulator to generate a power-supply voltage that tracks an envelope of an RF signal. The delays of the phase and envelope paths must be matched well to maintain a low Error Vector Magnitude (EVM) and high linearity, requiring additional circuitry. Envelope-tracking can be implemented by changing a bias current of a DA. In this case, the bias current is derived from the RF envelope by squaring the RF signal. Also, the envelope tracking method is a single-ended technique where the linearity of the envelope generating and supply modulation circuit directly affects the linearity performance of the power amplifier/drive amplifier.
ET methods are applied to RF circuitry that processes an I-Q combined and upconverted RF signal, which has a slowly-varying envelope. In addition, in an IQ upconverter, the individual I and Q baseband signal streams have more instantaneous signal swing variation than the RF envelope. Effectively, each I and Q stream has a larger PAPR as compared to the RF envelope. For example, while an RF envelope of a 2G GMSK signal is constant, the underlying I and Q streams have a PAPR of 3 dB.
In the related art, a rectification circuit and an envelope detection circuit are two separate circuits having two different circuit designs, where one circuit performs rectification while the other circuit performs envelope detection. The related art does not disclose one circuit that can perform both rectification and envelope detection. In addition, rectification circuits and envelope detection circuits receive a single-ended input and provide a single-ended output voltage.